Epitaxially-grown backward diode

ABSTRACT

A method of epitaxially growing backward diodes and diodes grown by the method are presented herein. More specifically, the invention utilizes epitaxial-growth techniques such as molecular beam epitaxy in order to produce a thin, highly doped layer at the p-n junction in order to steepen the voltage drop at the junction, and thereby increase the electric field. By tailoring the p and n doping levels as well as adjusting the thin, highly doped layer, backward diodes may be consistently produced and may be tailored in a relatively easy and controllable fashion for a variety of applications. The use of the thin, highly doped layer provided by the present invention is discussed particularly in the context of InGaAs backward diode structures, but may be tailored to many diode types.

PRIORITY CLAIM

This divisional application claims the benefit of priority to U.S.utility application Ser. No. 09/398,393, now U.S. Pat. No. 6,507,043,filed in the United States on Sep. 17, 1999, issued on Jan. 14, 2003,and titled “Epatially-Grown Backward Diode”.

TECHNICAL FIELD

The present invention relates to semiconductor devices, and moreparticularly to backward diodes that may be used for microwave detectionand mixing.

BACKGROUND OF THE INVENTION

The tunnel diode is a well-known semiconductor device thatconventionally includes two regions of heavily doped semiconductormaterial of opposite conductivity types separated by a relatively thinjunction which permits charge carriers to tunnel through upon theapplication of a suitable operating potential to the semiconductorregions. The p and n regions of tunnel diodes are so heavily doped thatthey are degenerate. At equilibrium, a portion of the valence band inthe p region of the diode is empty and part of the conduction band inthe n region is filled.

Adding a slight back bias brings some filled energy levels of thevalence band of the p region to empty energy levels of the conductionband of the n region and, consequently, electrons will flow from the pregion to the n region by the quantum-mechanical tunnel effect. Becausethe number of available valence band energy levels increases with backbias, and since the distance over which the electrons must tunneldecreases with increasing back bias, the back current increases veryrapidly with increasing back bias. This is known as the Zener effect.Since the direction of current flow is when the electrons flow from then region to the p region, the above current is negative.

A slight forward bias brings some levels of the filled part of theconduction band of the n region to empty levels of the valence band ofthe p region. In this situation, quantum-mechanical tunneling allowselectrons to flow from the n region to the p region, giving a positivecurrent that first increases with increasing back bias. When the filledpart of the conduction band of the n region is maximally aligned withthe empty part of the valence band of the p region, the current goesthrough a maximum. Subsequently, the current decreases with increasingforward bias, and should approach zero if the filled part of theconduction band of the n region lies opposite the energy gap of the pregion. When a yet larger forward bias occurs, electrons and holes areinjected over the barrier into the p and n regions, respectively,resulting in a rapid increase in current for increasing forward bias.Thus, the current-voltage has a negative conductance part in the forwardregion of the characteristic.

In practice, the majority of tunnel diodes are manufactured using one ofthe following techniques: (1) ball alloy, in which a small metal alloypellet containing a counter-dopant of high solid solubility is alloyedto the surface of a mounted semiconductor substrate (with high doping)in a carefully controlled temperature-time cycle under inert or hydrogengas, with the desired peak current level obtained by an etching process;(2) pulse bond, in which the contact and the junction are madesimultaneously when the junction is pulse-formed between thesemiconductor substrate and the metal alloy containing thecounter-dopant; or (3) planar processes, in which fabrication consistsgenerally of the use of planar technology including solution growth,diffusion, and controlled alloying.

Tunnel diodes originated with a device commonly known as an “Esakidiode”, which was described by Leo Esaki in “New Phenomenon in NarrowGermanium p-n Junctions”, Phys. Rev., 199, 603, published in 1958. Therehe described, while studying the internal field emission in degenerategermanium p-n junctions, an anomalous current-voltage characteristicobserved in the forward direction, which amounted to a negativeresistance region over part of the forward characteristic. Thischaracteristic was described through the use of a quantum tunnelingconcept and led the way to many devices based on its concept. Thetunneling time is very short, which permits the use of tunnel diodeswell into the millimeter-wave region.

If the doping levels of the p and n regions are decreased, the filledportion of the conduction band of the n region and the empty part of thevalence band of the p region become narrower. Consequentially, therewill be fewer energy levels from which tunneling under a forward biascan occur. Hence, the maximum in the forward current-voltagecharacteristic becomes lower and lower until it practically disappearsat a given doping level. At this point, the rectifying characteristicbecomes the opposite of that of a normal diode. Such a diode istherefore is known as a backward diode.

Backward diodes are utilized in applications such as microwave detectionand mixing. Their advantages include high speed, linear square lawbehavior, zero or small required bias, and temperature insensitivity.The standard backward diode is made from a heavily doped Ge p-nhomojunction as utilized in Esaki-type diodes. Ge diodes of this typeare manufactured utilizing crude alloying and diffusion techniques thatare difficult to control. Furthermore, the materials are notsufficiently stable to withstand normal temperatures when attaching todie, and epoxying is required. Therefore, it is an object of the presentinvention to provide a backward diode for which manufacture is easilycontrollable, which may be attached normally to a die, and which may beused as a more consistently manufacturable replacement for current Geback diodes.

References that provide further background regarding backward diodesinclude,

C. A. Burrus, IEEE Transactions on Microwave Theory and Techniques, p.357, September 1963, describing Ge back diodes and their characteristicsas detectors;

T. A. Richard, E. I. Chen, A. R. Sugg, G. E. Hofler, and N. Holonyak,Applied Physics Letters 63, p. 3613 (1993), describing GaAs backwarddiodes with a 100 Å In_(0.1)Ga_(0.9)As layer; and

W. L. Chen, G. O. Munns, X. Wang, and G. I. Haddad, Proceedings IEEECornell Conference, August 1995, p. 465 (1995), describing InGaAs (onInP) uniformly doped (n=p=5×10¹⁹ cm⁻³) for conventional Esaki tunneldiodes.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a method for epitaxially-growing abackward diode device including the steps of epitaxially growing ann-side including at least one n-doped layer; epitaxially growing a thin,heavily n-doped layer on the n-side; and epitaxially growing a p-sideopposite the n-side on the thin, heavily n-doped layer, said p-sideincluding at least one p-doped layer. Furthermore, the method ispreferably used when the at least one n-doped layer, the thin, heavilyn-doped layer, and the at least one p-doped layer are grown of asemiconductor material, and optimally when the semiconductor material isInGaAs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a band diagram for a conventional implementation of abackward Esaki diode;

FIG. 2 illustrates a current-voltage (I(V)) curve for a conventionalimplementation of a backward Esaki diode;

FIG. 3(a) provides a simulated band diagram for an InGaAs diodeproviding a p-side uniformly doped at 7×10¹⁸ cm⁻³, with a 100 Å layern-doped to 1×10¹⁹ cm⁻³ inserted at the junction, and wide contact layerswith p=7×10¹⁸, n=3×10¹⁸;

FIG. 3 (b) provides a simulated carrier concentration profile for anInGaAs diode providing a p-side uniformly doped at 7×10¹⁸ cm⁻³, with a100 Å layer n-doped to 1×10¹⁹ cm⁻³ inserted at the junction, and widecontact layers with p=7×10¹⁸, n=3×10¹⁸;

FIG. 4 provides a diagram showing the layer structure of the presentinvention;

FIG. 5 provides current-voltage (I(V)) curves for several test samplesof the present invention and for a conventional Ge structure;

FIG. 6 provides tabular data for the test samples of FIG. 5.

DETAILED DESCRIPTION

The present invention relates to backward diodes, as well as toapparatus incorporating them therein. The following description ispresented to enable one of ordinary skill in the art to make and use theinvention and to incorporate it in the context of particularapplications. Various modifications to the preferred aspect, as well asa variety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to other aspects. Thus, the present invention is not intendedto be limited to the aspects shown, but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

A band diagram of a conventional implementation of a typical backwardEsaki diode is shown in FIG. 1 and its current-voltage I(V) curve isshown in FIG. 2. The backward Esaki diode includes a conventionalheavily doped p-n junction of Ge or other semiconductor. The heavydoping bends the valence bands 100 and 102 sufficiently to allowelectrons in the n-doped side to tunnel through the relatively thinband-bending region into the p-doped side. If the doping is very heavyon both sides, a negative resistance peak 200 in the I/V curve isproduced for positive bias as the electrons tunnel from the n-doped sideto the holes in the p-doped side. The vertical arrows in FIG. 1 indicatethe direction of the shift of valence band edges 100 and 102 withpositive bias. For sufficient positive bias the electron energies aretoo high for tunneling into the hole states, and negative differentialresistance results, as demonstrated by the downward slope 202 withincreasing voltage in FIG. 2. For negative bias, if the doping is high,the band-bending region is short, and electrons from the p-doped side atenergies below the Fermi level 104 can tunnel from left to right intothe n-doped side. This current can be large and increases exponentiallywith reverse bias, as demonstrated by the I(V) curve to the left of theorigin 204 in FIG. 2.

If the p-type doping is not too large, the Fermi level 104, in FIG. 1,will be close to the valence band edge on the p-type side 102. In thissituation, there are relatively few hole states for the electrons totunnel into with forward bias. The peak current of the negativeresistance I(V) will be small, while the tunneling current in thenegative bias direction 204 is relatively unaffected and large, as shownin FIG. 2. The desirable characteristic is the highly non-linear I(V)near zero bias, represented by the origin in the I(V) curve shown inFIG. 2. This characteristic makes the backward diode an extremely usefuldevice for mixing and detecting of RF signals.

Ge diodes have become established as the most useful semiconductorchoice for backward diodes. This is mainly due to the band gap, about0.67 eV, at room temperature, which is small enough so that the amountof tunneling is large for small negative bias. At the same time it islarge enough to block normal p-n diode-type thermally activatedtunneling over the built-in junction voltage for small positive bias.Thus the desirable characteristic of large backward current and smallforward current for small bias is achieved.

Ge is rarely grown with modem epitaxial growth technology such asmolecular beam epitaxy (MBE) due to the rarity of other Ge semiconductordevice applications. InGaAs in particular, by contrast, has increasingapplication for type III-V semiconductor circuits and InGaAs epitaxialgrowth facilities are easily accessible. InGaAs grown with lattice matchto InP is a natural substitute for Ge, as it has a small roomtemperature band gap of 0.75 eV. Attempts have been made to grow heavilydoped p-n junctions of InGaAs in order to duplicate the I(V) of acomparison Ge diode. Although backward diodes were easily achievable,initial results were not as good as the comparison Ge diode. Either highbackward currents or low forward currents could be achieved depending onthe particular sample, but not both in the same sample. This problem isattributable to two features. First, the slightly greater band gap ofthe InGaAs reduces the backward current compared to Ge. Second, InGaAsis a direct semiconductor, which causes it to have a significantlysmaller density-of-states conduction band effective mass than Ge; 0.043versus 0.22 in units of the free electron mass. This means that for asimilar n-type doping, the Fermi level in InGaAs is significantly higheras compared with Ge. This results in an extended positive voltage rangein which the tunneling current increases, i.e., a high peak voltage (seeFIG. 2), with an undesirable linear I(V) near the origin.

To overcome this limitation, the ability to tailor the InGaAssemiconductor layers on a nanometer scale is required. Control of thep-doped side is not as critical as that of the n-doped side due to thehigh density-of-states effective mass of holes as compared withconduction electrons, so it may be kept as highly doped and homogeneous.In the present invention, a thin, highly doped region in the n-side nearthe p-n junction is used to increase the local electric field, while alower doped region further away keeps the Fermi level adequately low.This significantly enhances the backward tunneling current, while onlymoderately increasing the undesirable forward current. The highly dopedregion must be thin enough so that the Fermi level does not re-acquirethe value associated with the bulk material with that correspondingdoping. FIG. 3(a) and FIG. 3(b) show the results of a simulation of theband profile and carrier concentration profile, respectively, where thep-side is uniform with a doping level of 7×10¹⁸ cm⁻³, a 100 Å layern-doped to 1×10¹⁹ cm⁻³ is present at the junction, and a wide contactlayer n-doped with to a doping level of n=3×10¹⁸ cm⁻³ produces asufficiently low Fermi level for the contact layers. The effect of the100 Å region can be seen in the band profile in that it steepens thevoltage drop at the junction and thereby increases the electric field.The Fermi level relative to the conduction band is slightly increased,but not significantly. The plot of the concentration versus distanceshows a corresponding increase in the number of conduction electrons inthe added thin layer. Conventional diode manufacturing techniques affordinsufficient control to produce a consistent thin layer. Therefore,epitaxial-growth techniques such as MBE provide the means forconsistent, controlled production of a thin layer.

FIG. 4 provides a diagram showing the layer structure of an aspect ofthe present invention utilizing a thin layer similar to that describedrelative to FIG. 3(a) and FIG. 3(b). As can be seen from the diagram,the fabrication begins with a substrate 400 of InP n+ or othersemi-insulating material having a thickness that may be chosen suitableto the particular application. Next, a first InGaAs layer 402 isdeposited and n+ doped to generally between 1×10¹⁸ to 1×10¹⁹ cm⁻³,typically 3×10¹⁸ cm⁻. The thickness of the first InGaAs layer is shownin the figure as typically about 5000 Å. Subsequently, a second, thinand highly n+ doped InGaAs layer 404 is deposited, with the n+ dopinglevel generally between 3×10¹⁸ and 3×10¹⁹ cm⁻³; typically 1×10¹⁹. It isimportant that the doping level of the second InGaAs layer is greaterthan that of the first InGaAs layer. The thickness of the second InGaAslayer is generally between 25 Å and 200 Å, typically 100 Å. Next, athird InGaAs layer 406 is deposited with a p+ doping generally between5×10¹⁸ and 2×10¹⁹ cm⁻³, and typically 1×10¹⁹ cm⁻³, and with a thicknessgenerally ranging from from 200 Å to over 1000 Å, and typically 500 Å.Subsequently, a fourth InGaAs layer 408 is deposited with a p+ dopinggenerally ranging from 2×10¹⁹ to 5×10⁹ cm⁻³, and typically 3×10¹⁹ cm⁻³and a thickness generally ranging from 100 Å to 1000 Å, and typically200 Å. Finally, the structure is etched to provide the desiredelectrical isolation and contact regions, and contacts 410 and 412 aredeposited onto the structure. Note that FIG. 4 provides a cutaway viewof what typically is formed as a circular mesa-type structure, and thatcontact 412 would generally be formed as a ring. Utilizingepitaxial-growth techniques such as molecular beam epitaxy (MBE) andmetal-organic molecular beam epitaxy (MOMBE) to produce the InGaAsstructure of FIG. 4 allows for improved control over the manufacturingprocess, resulting in a more consistent diode. Without the controlafforded by these techniques, tailoring the second InGaAs layer inparticular would not be feasible. Thus, it is important that the secondInGaAs layer be formed by an epitaxial-growth technique or a methodwhich achieves a similar result. Furthermore, although InGaAs has beenchosen for the material structure of this example, there are othermaterial structures to which it could apply.

FIGS. 5 and 6 provide measurements from samples including three InGaAsfabricated diodes and a Ge diode. FIG. 5 provides current-voltage curvesgenerated for each of the samples. FIG. 6 provides pertinent numericaldata regarding each of the samples in units contained in the same dataas the current-voltage (I(V)) diagram of FIG. 5. In FIG. 6, V_(v)provides the voltage in volts, V, at the local minimum of the valleytoward the right hand side of the figure for each samples, J_(p)provides the current in Amps/cm² at the local maximum of the currentpeak located near the middle of the chart for each of the samples, R_(j)provides the resistance in ohms, Ω, at zero bias, and γ is the curvaturecoefficient, where$\gamma = \frac{\frac{\partial^{2}I}{\partial V^{2}}}{\frac{\partial I}{\partial V}}$

The last column in FIG. 6, labeled d provides the thickness, inangstroms, Å, of a thin InGaAs layer with a doping level of 1×10¹⁹ cm⁻³corresponding to the InGaAs layer 304 of FIG. 3. Of the samples, aconventional diode, 2025, was fabricated without the thin InGaAs layer,providing a curvature coefficient γ of approximately 13 and a lowcurrent density of 60, resulting in a relatively slow operational speed.The best diode with the thin InGaAs layer was 2026, which had a highercurrent density J_(p) and a lower curvature γ than 2089, demonstratingthe ability to trade off desirable features. Sample 2026 also provided alower resistance R_(j) of approximately 155 Ω. It is important to notethat these samples are provided merely for illustrative purpose and notto infer any limitations to the present invention.

A figure of particular importance for the voltage sensitivity of squarelaw power measurement applications is γ, the I(V) curvature divided bythe slope. Values comparable to the Ge diode were obtained. Without theextra thin layer, γ was limited to less than 20. Thus, the InGaAs diodeof the present invention which provides a controllable profile designedto maximize curvature near the origin of the I(V) curve is useful formany applications, especially those requiring highly specificcharacteristics such as detectors.

What is claimed is:
 1. A method for epitaxially-growing a backward diodeincluding the steps of: growing an n-side including at least one n-dopedInGaAs layer; growing a thin, heavily n-doped InGaAs layer on then-side; and growing a p-side opposite the n-side on the thin, heavilyn-doped InGaAs layer, said p-side including at least one p-doped InGaAslayer.
 2. A method for epitaxially-growing a backward diode as set forthin claim 1 further including the step of: forming electrical contacts onthe n-side and p side.
 3. A method for epitaxially-growing a backwarddiode as set forth in claim 2, wherein the step of growing a thin,heavily n-doped InGaAs layer on the n-side includes the following steps:growing a thin layer on the n-side to a depth of approximately between25 and 200 Å; and doping the thin layer on the n-side to a concentrationapproximately between 3×10¹⁸ and 3×10¹⁹/cm³ such that the dopingconcentration of the thin layer is greater than that of the mostproximate one of the at least one n-doped InGaAs layer of the n-side. 4.An epitaxially-grown backward diode fabricated according to the methodof claim
 2. 5. An epitaxially-grown backward diode fabricated accordingto the method of claim 1.